<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
	<style type="text/css">
		
		#hd{
			width: 300px;
			height: 150px;
			background: dodgerblue;
			animation: run 3s infinite;
		}
		
		/*定义运动规则*/
		@keyframes run{
			0%{
				width: 300px;
			}
			50%{
				width: 900px;
			}
			100%{
				width: 300px;
			}
		}
		
		
		
		
		
	</style>
	</head>
	<body>
		
		<div id="hd"></div>
		
		
	</body>
</html>
